98 



VERTEBRATE ZOOLOGY 



example of this group is the sun-fish, representing that vast assem- 

 blage of modern fishes, the Acanthopterygii, which includes more 

 species than all other groups of fishes combined. Extreme cases of the 

 laterally compressed type are seen in the "Head-Fishes" and in Zan- 

 dus (Fig. 86), which are about as high as they are long and are veiy 

 much compressed. This foreshortening of the long axis accompanied 



Fig. 52. — Origin and Adaptive Hadiatiun of the l''islics. Dulled arca.s rep- 

 resent groups still existing; black areas represent extinct groups. (After Osborn 

 and Gregory.) 



by corresponding increase in height may be taken as a sjmiptom of 

 racial senescence; for, according to this view, there has lieen a retarda- 

 tion in growth vigor down the principal axis accompanied by a marked 

 acceleration of growth in the secondary (dorso-ventral) axis. 



The bottom-feeding type (Fig. 53, j, k, I) is one that involves many 

 grades and types of specialization. In general, Ijottom feeders are 

 depressed dorso-ventrally and are broad bilaterally. Common rep- 

 resentatives of this adaptive ass(nnl>lage are the skates and rays, the 

 extinct ostracoderms, and several types of bottom-feeding teleosts. 

 They are essentially the oldest or most senescent of (he fish types, 

 as evidenced by the comparative suppression of the primary or longi- 

 tudinal axis and the secondary or dorso-ventral axis l)y the tertiary 

 or bilateral axis. 



Thus it will be seen that the fishes, although in a less varied habitat 



